<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #previewCaptureImg{
      display: block;
      width: 50%;
      margin: 0px auto;
    }
  </style>
</head>
<body>
  <div>
    <button id="downloadFile">下载图片</button>
  </div>
  <img src="" id="previewCaptureImg">
</body>
<script src="preview.js"></script>
</html>